Second-order rate constants (kH) for the oxidation of substituted benzyl alcohols via hydride-transfer to xanthylium ion (Xn+), 9-phenylxanthylium ion (PhXn+), and thioxanthylium ion (TXn+), and for the oxidation of benzyl alcohol by 9-arylxanthylium ions (ArXn+) and 9-arylthioxanthylium ions  (ArTXn+) in acetonitrile at 60oC were determined. The direct oxidation products of alcohols are a�hydroxy-carbocations and those of the reduction of cations are the neutral dihydro products.  Hammett correlation for the oxidation of substituted alcohols gave the reaction constant ? = -2.66 (PhXn+), -2.42 (Xn+) and -2.62 (TXn+), while the reduction of the 9-aryl cations gave ?+ = 0.47 (ArXn+) and  ?+ = 0.23 (TArXn+).  The results suggest imbalanced transition states in which the C-H bonds from alcohol are largely broken but those from the cations are formed to a smaller extent.  The results indicate that the transferring hydrogen carries partial negative charge.

Transition state structure of the alcohol oxidations by NAD+ analogs

245th National Meeting (2013)

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